This invention relates to a DC-AC converter for igniting and supplying a gas discharge lamp, e.g. a fluorescent lamp, the converter having two input terminals intended to be connected to a d.c. voltage source, the input terminals being connected together in series by an arrangement of at least a first semiconductor switching element, a capacitor and a load circuit comprising at least an induction coil and the gas discharge lamp. The capacitor and load circuit are shunted by a second semiconductor switching element provided with a control circuit comprising at least a starter circuit and a resonant circuit. The resonant circuit includes the parallel arrangement of the transformer primary winding and a capacitor in one branch and the gas discharge lamp in the other branch.
A DC-AC converter of this type is known from U.S. Pat. No. 4,415,838 and U.S. Pat. No. 4,748,383. The undimmed lamp situation is concerned in this case. In this known converter a transformer is present in the load circuit (in which the lamp is incorporated). This transformer has two secondary windings which form part of the control circuits of the semiconductor switching elements. The switching elements are rendered alternatively conducting and non-conducting by means of the transformer and the control circuits respectively. This known converter is designed for an electrodeless low-pressure gas discharge lamp.
However, a drawback of the known circuit is that in order to start a gas discharge lamp, e.g. a fluorescent lamp, a much higher voltage needs to be supplied to the lamp and hence the voltage across the resonant circuit which is incorporated in the series arrangement is much higher than the operating voltage. This results in a potential risk to the semiconductor switching elements. It has also been found that when the above mentioned arrangement is used for running multiple lamps with the same DC-AC converter a high current through one induction coil which is incorporated in the series arrangement with the resonant circuit and the lamps is needed to be able to supply enough power for the lamps. This is a drawback because such circuits cannot easily be used universally with lamps having different power ratings. The known circuit doesn't allow the current supplied to the lamp to be set to a predetermined value during operation of the lamp, this would offer a longer lamp life because the current through the lamp increases due to ageing, or in the case of a low pressure vapour discharge lamp, operation at a relatively hot location.